A typical motor vehicle clutch control system moves the clutch from a disengaged position to an incipient torque position or touch point at a relatively fast rate. If the clutch actuator overshoots this initial torque position at the fast rate, the resulting buildup of clutch torque may be excessive and cause the vehicle to lurch or bounce, such that a smooth start is not achieved. It is therefore important to accurately identify the touch point.
It has been proposed to optimize the positioning of a motor vehicle clutch at the incipient torque position utilizing a variety of sensed parameters including engine speed and transmission input speed. See for example U.S. Ser. No. 817,161 filed Jan. 6, 1992, now U.S. Pat. No. 5,332,074, assigned to the assignee of the present invention, as well as U.S Pat. Nos. 5,176,234; 5,029,678; and 4,678,069 all of which are incorporated herein by reference.
In systems where speed sensors such as those of the variable reluctance type are used, the sensor output signal drops to zero at some finite "dropout" speed greater than zero, for example 180 rpm for a typical sensor. When attempting to locate the incipient torque position of the clutch, equivalent to sensing the first rotation of the transmission input shaft, some method of compensating for lack of information from the speed sensor at the touch point must be employed or error will be introduced.